Magnetic resonance imaging (MRI) is a relatively new technology which can be used to display two-dimensional image slices of the human body on a video screen. The technology offers an advantage over conventional X-ray techniques in that the patient is not exposed to harmful ionizing radiation and the adverse effects therefrom. MRI systems can also be used to observe many different types of atoms and can identify the chemical environment of the atoms.
In operation, a magnetic resonance imaging system uses a strong main magnetic field to selectively orient atoms having an odd number of protons in their nuclei. A second magnetic field oscillating at a radio frequency rate, usually applied at right angles to the main field, is then used to flip these nuclei into an inverted state. When the applied magnetic field is subsequently removed, the nuclei relax from their inverted state, and in doing so, radiate energy in the form of weak but detectable electromagnetic waves. The resulting signals are then received and used by the MRI system to generate a two-dimensional display of a specimen's composition.
To accomplish all this, magnetic resonance imaging systems generally comprise a main magnetic field generator, a radio frequency excitation means, control/display circuitry, and a magnetic resonance receiving probe. The main magnetic field generator provides a main magnetic field along a Z-axis for the purpose of aligning individual atoms. The radio frequency (RF) excitation means selectively excites the nuclei of these atoms, and subsequently allows the nuclei to relax. The resulting electromagnetic signal produced by the relaxing nuclei is then received by the receiving probe and processed by the control/display circuitry to produce an image representative of the specimen's composition.
The received electromagnetic signals are in the form of a circularly polarized or rotating magnetic field, having an axis of rotation aligned with the main magnetic field of the MRI system. By using a receiving probe capable of constructively adding the two perpendicular components of the rotating magnetic field, a stronger signal can be extracted by the MRI system. Receiving probes of this type which measure two perpendicular components of a magnetic resonance signal are well known in the art and are commonly referred to as quadrature probes or quadrature coils.
For an ideal quadrature probe structure, isolation between the individual coils must be maintained, as well as the perpendicular relationship between their respective field vectors. Maintaining isolation along with a perpendicular relationship, tends to optimize the signal-to-noise ratio of the receiving probe and thereby improves the overall signal-to-noise ratio of the MRI system.
Quadrature probes, in the past, have been devised from a variety of individual coil configurations, including generally cylindrical shapes as well as planar structures. Regardless of which configuration is used, it has always been a difficult undertaking to maintain the isolation between quadrature coils while at the same time, maintaining the perpendicular relationship between their magnetic vectors.
One such attempt at providing maximized coil isolation contemporaneously with maintenance of a perpendicular magnetic relationship, is disclosed in Arakawa U.S. Pat. No. 4,752,736. See also, Siebold U.S. Pat. No. 4,467,282, one of the early quadrature coil patents disclosing a uniform symmetrical volume coil. In particular, U.S. Pat. No. 4,752,736 discloses a rather complex coil structure utilizing a series of breaks, conductive bridges, and capacitive elements to provide isolation between the individual coils of the system. This complex coil configuration is of the volume type and, therefore, completely surrounds the specimen or patient being analyzed.
Another quadrature coil arrangement is disclosed in Fehn U.S. Pat. No. 4,707,664. According to the patent, a surface coil configuration is disclosed and consists of two separate coils. Each coil is rigidly mounted to a separate quasi-cylindrical substrate. One substrate and its respective coil is circumferentially surrounded by the other substrate and coil such that the inner substrate is retained. In addition to being retained, the inner substrate can be adjusted to assume any angular orientation with respect to the other substrate. In this way, the angle between the individual coils can be adjusted to maximize isolation, and thereby, increase the overall signal-to-noise ratio. If the isolation must be adjusted further, a series of isolating capacitive couplings can be connected across predetermined portions of the surface coils.
Planar quadrature coil systems are disclosed in Boskamp U.S. Pat. No. 4,839,595 and Mehdizadeh et al. U.S. Pat. No. 4,918,388. These coil systems comprise two coil loops mounted side-by-side (Boskamp) or on top of each other (Mehdizadeh) in a planar dielectric sheet.